The present invention relates to systems for, and methods of, recovering digitally modulated television signals and, more particularly, to a dual mode QAM/VSB receiver system for recovering quadrature amplitude modulated or vestigial sideband modulated signals.
Modern digital telecommunication systems are operating at ever-increasing data rates to accommodate society""s growing demands for information exchange. However, increasing the data rates, while at the same time accommodating the fixed bandwidths allocated by the Federal Communications Commission (FCC), requires increasingly sophisticated signal processing techniques. Since low cost, small size and low power consumption are portent in the hardware implementations of such communication systems, custom integrated circuit solutions are important to achieving these goals.
Next generation digital television systems, such as cable transported television (CATV) and high-definition television (HDTV) rely on telecommunication transceivers to deliver data at rates in excess of 30 megabits per second (30 Mb/s). The ATSC A/53 Digital Television Standard, was developed by the xe2x80x9cDigital HDTV Alliancexe2x80x9d of U.S. television vendors, and has been accepted as the standard for terrestrial transmission of SDTV and HDTV signals in the United States. The ATSC A/53 standard is based on an 8-level vestigal sideband (8-VSB) modulation format with a nominal payload data rate of 19.4 Mbps in a 6 MHz channel. A high data rate mode, for use in a cable television environment, is also specified by the standard. This particular mode, defined in Annex D to the ITU-T J.83 specification, utilizes a 16-VSB modulation format to provide a data rate of 38.8 Mbps in a 6 MHz channel.
Transmission modes defined in ITU-T J.83 Annex A/C are used primarily outside the United States for digital cable television transmission. The transmission modes supported by this specification have been adopted in Europe as the Digital Video Broadcast for Cable (DVB-C) standard, and further adopted by the Digital Audio-Video Council (DAVIC) with extensions to support 256-QAM modulation formats.
Beyond these divergent requirements, the ITU-T J.83 Annex B standards define the dominant methodology for digital television delivery over CATV networks in the United States. It has been adopted as the physical layer standard by various organizations including the SCTE DVS-031, MCNS-DOCSIS and the IEEE 802.14 committee.
Given the implementation of multiple modulation techniques in the various adopted standards, there exists a need for a television receiver system capable of receiving and demodulating television signal information content that has been modulated and transmitted in accordance with a variety of modulation formats. In particular, such a system should be able to accommodate receipt and demodulation of at least 8 and 16-VSB modulated signals in order to support US HDTV applications, as well as 64 and 256-QAM modulated signals, for European and potential US CATV implementations.
The present invention is directed to digital data communication systems and methods for operating such systems in order to synchronize a receiver""s timebase to a remote transmitter""s. Carrier frequency and symbol timing information is recovered from a pilot (unsuppressed carrier) signal that is inserted into a VSB spectrum. Unlike conventional timing recovery systems, which recover timing information from the segment sync signal, that is provided at the end of every line of 828 symbols, and is specifically designed to facilitate timing recovery.
In a first aspect of the invention, a digital communication system includes an analog front end which receives an input spectrum in an intermediate frequency. The input spectrum includes an inserted pilot signal, representing a pre-determined frequency component. First and second nested tracking loops are provided, with the first loop acquiring carrier frequency lock in operative response to the predetermined frequency component. The second loop provides a signal adapted to position the input spectrum at a predetermined location relative to baseband in operative response to the predetermined frequency component. A third tracking loop is coupled so as to define a symbol timing parameter in operative response to the same predetermined frequency component. The digital communication system includes an equivalent filter which operates on the received spectrum in order to define a pair of symmetric signals, each of the symmetric signals centered at the characteristic frequency of the predetermined frequency component when the received spectrum is at baseband.
In an additional aspect of the invention, the equivalent filter is constructed of a first, high pass filter, having a lower cut-off frequency which is related to the system""s sampling frequency. The equivalent filter further includes a second, low pass, filter which has an uppercut-off frequency bearing the same relationship to the sampling frequency as the high pass filter. The first and second filters therefore define an equivalent band pass filter having symmetric passband regions centered about a frequency bearing the same relationship to the sampling frequency. When the received spectrum exhibits a raised cosine response characteristic, the passband regions incorporate the transition regions of the high and low pass filtered spectra.
The pilot signal is provided at a characteristic predetermined frequency fc. In one particular embodiment of the invention, the sampling frequency fs is chosen such that the pilot frequency is equal to fs/4. The high pass filter accordingly has a lower cut-off of fs/4 and a passband center of about fs/2. The low pass filter has an upper cut-off of about fs/4, the equivalent filter passband is thereby centered at a frequency of fs/4. Since one-fourth the sampling frequency, i.e., fs/4, is designed to be equal to the pilot frequency fc, the equivalent filter passband regions are centered at fc when fc=fs/4.
In a further aspect of the invention, an equivalent filter passband signal is provided to a phase/frequency detector which is constructed so as to determine whether the pilot signal is centered in the passband region. An oscillator circuit develops a timing reference signal having a frequency related to the sampling frequency, the oscillator circuit increasing or decreasing the timing reference signal frequency in operative response to the position of the pilot signal with respect to the passband region center.
A system according to the invention might thus be characterized as including a filter circuit for isolating an inserted pilot signal; a detector circuit coupled to receive the isolated pilot signal and compare its frequency value to a predetermined frequency; a frequency reference generation circuit for increasing or decreasing a reference frequency based upon the comparison result and a symbol timing circuit, defining consecutive symbol occurrence intervals, in operative response to the reference frequency. The symbol timing circuit operates at a sample frequency having an integer relationship to the pilot signal. The pilot signal will appear at a correct location in the spectrum if the sampling frequency is correct. The pilot signal will be shifted away from its expected frequency location, in a first direction, if the sampling frequency is too high, and will be shifted away from the expected frequency location, in the other direction, if the sampling frequency is too low.
The equivalent filter passband signals, containing an augmented pilot, are provided as inputs not only to a symbol timing loop, but also to first and second carrier recovery loops. Carrier recovery and symbol timing is therefore performed in operative response to the same augmented pilot input signal.